1. Field of the Invention
The present invention relates to a fluorescent display tube and more particularly to a fluorescent display tube having a built-in driving semiconductor element in a vacuum container.
2. Description of the Related Art
Fluorescent display tubes of the sort described above have been known to include a metal shield to prevent a driving semiconductor element from being damaged at the time of getter flashing which is formed integrally with a filament support.
More specifically, as shown in FIG. 1(a), a shield 1 has outward extending eaves 3 of the minimum size necessary to cover a driving semiconductor element 2 and a wire bonding area. The shield connections 5 for joining the filament support 4 to the shield 1 are in the form of a plurality of fine wires extending from opposing sides of the shield 1 to each lead 6. Each shield connection 5 has a width ranging from about half the thickness to about the same thickness of the metal sheet blank from which the metal shield is formed. The shield connections 5 comprise a plurality of fine wires which are subjected to plastic deformation to provide the solid form shown in FIG. 1(b). The solid form is composed of fixing parts 7 for supporting filaments, a getter fitting part 8, the shield 1, the shield connections 5 and the outward extending eaves 3. Any other part which is not a solid body in the shield support structure is made flat so as to reinforce the structure of the filament support 4. Before the fluorescent tube is vacuum sealed, a part of the filament support 4 in combination with the lead 6 is fitted into the sealing area 9 between the cover glass and the anode substrate which form the vacuum container. In this way, a fluorescent display tube is produced.
The fluorescent display tube needs a getter film to improve and maintain the degree of the enclosed vacuum. As the getter film is impenetrable to light, the common practice has been to form the getter film not within the domain of the display portion (not shown) but on the side of the filament support 4 having the shield 1 so as not to affect the visual display.
As set forth above, the getter film essential for improving and maintaining the degree of the vacuum inside the fluorescent display tube is formed by locally heating the getter by induction heating to evaporate and deposit the getter material inside the tube. However, since each shield connection 5 located close to the getter fitting part 8 forms a closed circuit as shown in FIGS. 1(a) and 1(b), there is a possibility that the shield connections 5 will be heated by the high-frequency induction current which flows at the time the getter is locally heated by high-frequency induction heating.
Although the filament support 4 is held between the cover glass and the anode substrate for sealing purposes, both the filament support 4 and the shield connections 5 tend to float off the anode substrate (not shown) because, with the exception of the sealing area 9, there exists a small gap with the anode substrate. Consequently, the heat dissipation property of the shield connection 5 in the conventional shield support structure is poor when it is heated by high-frequency induction heating, and the shield connection 5 may ultimately fuse with the rise in temperature. This has been the reason for the low production yield of the fluorescent display tubes.